Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (277)	$ (73)	$ (283)	$ (283)	$ (95)
Other Comprehensive Income before Reclassifications	(46)	(13)		(43)	17
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	24	(13)		27	(21)
Other Comprehensive Income (Loss), net of tax	(22)	(26)		(16)	(4)
Accumulated Other Comprehensive Income (Loss), Ending Balance	(299)	(99)	(277)	(299)	(99)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	1	1	10	10	(2)
Other Comprehensive Income before Reclassifications	0	2	1	1	(2)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	(1)	(10)	(10)	6
Other Comprehensive Income (Loss), net of tax	0	1		(9)	4
Accumulated Other Comprehensive Income (Loss), Ending Balance	1	2	1	1	2
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(395)	(232)	(411)	(411)	(238)
Other Comprehensive Income before Reclassifications	0	0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	9	3		25	9
Other Comprehensive Income (Loss), net of tax	9	3		25	9
Accumulated Other Comprehensive Income (Loss), Ending Balance	(386)	(229)	(395)	(386)	(229)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	117	158	118	118	145
Other Comprehensive Income before Reclassifications	(46)	(15)		(44)	19
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	15	(15)		12	(36)
Other Comprehensive Income (Loss), net of tax	(31)	(30)		(32)	(17)
Accumulated Other Comprehensive Income (Loss), Ending Balance	86	128	117	86	128
Power [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(223)	(44)	(228)	(228)	(63)
Other Comprehensive Income before Reclassifications	(43)	(12)		(40)	15
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	21	(15)		23	(23)
Other Comprehensive Income (Loss), net of tax	(22)	(27)		(17)	(8)
Accumulated Other Comprehensive Income (Loss), Ending Balance	(245)	(71)	(223)	(245)	(71)
Power [Member] | Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	2	2	11	11	(1)
Other Comprehensive Income before Reclassifications	0	2		1	(2)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	(1)		(10)	6
Other Comprehensive Income (Loss), net of tax	0	1		(9)	4
Accumulated Other Comprehensive Income (Loss), Ending Balance	2	3	2	2	3
Power [Member] | Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(337)	(199)	(351)	(351)	(204)
Other Comprehensive Income before Reclassifications	0	0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	7	2		21	7
Other Comprehensive Income (Loss), net of tax	7	2		21	7
Accumulated Other Comprehensive Income (Loss), Ending Balance	(330)	(197)	(337)	(330)	(197)
Power [Member] | Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	112	153	112	112	142
Other Comprehensive Income before Reclassifications	(43)	(14)		(41)	17
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	14	(16)		12	(36)
Other Comprehensive Income (Loss), net of tax	(29)	(30)		(29)	(19)
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ 83	$ 123	$ 112	$ 83	$ 123